The present invention relates generally to dimensional metrology and, more specifically, to large volume physical measurement of three dimensional (3D) objects.
Dimensional Metrology is the science of calibrating and using physical measurement equipment to quantify the physical size of, or distance from, any given object. Inspection is a critical step in product development and quality control.
Dimensional Metrology requires the use of a variety of physical scales to determine dimensions, with the most accurate of these being holographic etalons or laser interferometers. The realization of dimensions using these accurate scale technologies is the end goal of dimensional metrologists.
Modern measurement equipment include hand tools, Coordinate-Measurement Machines (CMMs), machine vision systems, laser trackers, and optical comparators. A CMM is based on CNC technology to automate measurement of Cartesian coordinates using a touch probe, contact scanning probe, or non-contact sensor.
Optical comparators are used when physically touching the part is undesirable. Optical comparators can now build 3D models of a scanned part and internal passages using x-ray technology.
Furthermore, optical 3D laser scanners are becoming more common. By using a light sensitive detector (e.g. digital camera) and a light source (laser, line projector) the triangulation principle is employed to generate 3D data, which is evaluated in order to compare the measurements against nominal geometries either in a scale drawing or CAD Model.
Large volume metrology examples include precision measurement of aircraft and spacecraft, energy generation structures and devices, and large manufacturing and assembly facilities.
The CMM is a very powerful measuring device used in dimensional metrology because it simultaneously produces coordinates of a point on the object being measured based on a reference location of the CMM using a suitable coordinate system like the three orthogonal axis Cartesian coordinates X, Y, and Z having a common reference origin.
The laser tracker is a popular portable CMM that can calculate X,Y,Z coordinates for any point on an object. This is accomplished by measuring the distance between the tracker and each target point with a laser and combining it with the horizontal and vertical angles of the laser pointing device embodied in the tracker using a common reference coordinate systems for all points in the measurement survey.
An optical target in the exemplary form of a Spherically Mounted Retro-reflector (SMR) is placed at the desired point on the object for the laser tracker to precisely determine laser range and fix horizontal and vertical angles of the emitted laser beam in the pointing device.
Other portable CMMs include theodolites, robotic total stations, and a system of camera photos called photogrammetry. They all require Line of Site (LOS) between the portable CMM and the target point on the object they are measuring.
Since ultimately all the desired points measured on the 3D object need to be plotted in their exact relationship with each other in a suitable coordinate system, and because the CMM will most likely not have visibility on all the desired points from one location, the LOS requirement becomes a significant problem.
In large volume metrology, the object being surveyed is typically large in three dimensions and typically complex in configuration, and may therefore include a significant number of recessed or obstructed target points hidden from LOS view of the CMM within the full complement of desired survey locations or points.
However, because this type of CMM is portable, the CMM can be relocated to a new LOS reference location, or a second CMM may be used, for providing LOS measurements of survey points previously hidden at the first CMM location. CMM measurements from both viewing or source locations will therefore include both survey points with LOS coordinate measurements thereof, and other survey points hidden from LOS view of the differently located CMMs.
Since the two CMM viewing locations will have different coordinate references, a mathematical work-around to the LOS requirement, such as least squares optimization, may be used to mathematically tie together the measured coordinates based on some of the common survey points having LOS visibility from both CMM viewing locations to establish a common coordinate reference system for all measured points from both viewing locations.
Other solutions for measuring hidden points lacking LOS visibility include special optical targets cooperating with the CMM that include touch probes that can reach the hidden points while at least some portion of the probe remains within LOS visibility of the CMM.
However, such optical targets probes can have various configurations including different benefits and different problems in measuring the hidden survey point.
Significant to large scale dimensional metrology is the typical requirement for precision measurement of the 3D object coordinate locations X,Y,Z within very small dimensional tolerances of about plus/minus 0.6 mils (0.0006 inches or 15 microns), for example.
The typical laser tracker CMM can achieve this high precision; and highly specialized optical targets may be used therewith for matching such high precision based on different technologies having different problems and different benefits, and at correspondingly different cost.
Various optical targets and probes are known for various fields of endeavor including land surveying, and vary substantially in configuration and operation, with correspondingly different accuracy of measurement.
Fundamental to metrology are the typical six degrees of freedom (DOF) associated with 3D objects, which can be measured in a suitable coordinate system such as the exemplary six-axis Cartesian coordinate system introduced above. Three orthogonal linear axes X, Y, and Z extend outwardly from a common origin for defining linear position therefrom; and three angular or rotary axes A, B, C define angular position or attitude around the corresponding linear axes, commonly known as roll, pitch, and yaw.
Various technologies are commonly known for measuring linear position and angular attitude with varying degrees of complexity and accuracy. And, such various technologies may be combined in various manners for various benefits.
Many common measuring technologies are based on optical measurements having various optical encoders or camera systems, which require LOS. Other technologies include the Global Positioning Satellite (GPS) system commonly used in navigation for measuring or determining location based on longitude and latitude positions, but subject to the substantial problem of GPS signal loss.
Still other technologies include the Inertial Measurement Unit (IMU) also commonly used in navigation in which cooperating accelerometers and gyroscopes measure relative movement of the IMU in the six DOF, but subject to the also significant problem of inherent temporal drift errors.
All such measuring technologies have different capabilities and different problems, and correspondingly different costs.
For example, fundamental to IMUs is the significant drift errors inherent therein which increase exponentially, or quadratically, with time. Accordingly, commercial inertial sensors based on IMUs have a six-order magnitude difference in price and performance in different configurations or grades thereof.
Four IMU grades include automotive & consumer; industrial; tactical; and marine & navigation having correspondingly decreasing drift errors resulting in horizontal position errors of about 7900 km/hr, 190 km/hr, 19 km/hr, and 1.6 km/hr, respectively, with cost ranging from low for consumer grade to exceedingly high for the marine grade.
However, one particular advantage of IMUs is their dead-reckoning capability to measure both linear and angular positions without regard to the loss of LOS or GPS signal problems. Another particular advantage of IMUs is modern advancements thereto in which the size, cost, and drift errors of IMUs continue to decrease.
Accordingly, one object of the present invention is to provide improved large volume dimensional metrology of an object.
Another object of the invention is to provide an improved method for measuring location of one or more of the full complement of survey points having blocked LOS in a measurement survey of the object.
Another object of the invention is to provide location measurement of the hidden point with preferential precision thereof.
Another object of the invention is to provide an improved method and system for conducting large volume dimensional metrology having reduced complexity and cost.